1. Field of the Invention
This invention relates generally to electronic tuning systems, and more particularly to a resonant tuning system with dynamic impedance matching.
2. Description of the Related Art
Magnetic fields are used in many electronic systems for a variety of purposes such as Electronic Article Surveillance (EAS), Radio Frequency Identification (RFID), metal detectors, magnetic imaging systems, remote sensing, communications, etc. In these various electronic systems, a magnetic coil may be used as a transmitter to project a magnetic field into a desired sensing region. More particularly, for the transmitters, a highly efficient method for generating magnetic fields includes the use of a series resonant LCR circuit that presents a low impedance to the transmitter at the transmit frequency. To achieve high magnetic field levels from the antenna of the transmitter, it is desirable for the transmitter to deliver high currents to the antenna coil. Therefore, to achieve high performance, it is desirable to maximize the current delivered from the transmitter into the coil.
One method for maximizing the current delivered from the transmitter is to use a LCR circuit with a high quality factor (Q). This high Q may be accomplished by increasing the inductance of the antenna coil and by reducing the total series resistance of the circuit. Increasing the Q of the LCR circuit also increases the need for tuning, and more particularly, fine tuning of the resonant frequency of the LCR circuit. Due to tolerances in design and the resonant capacitor in the circuit, the circuit is tuned to match the natural resonant frequency of the LCR circuit to the frequency of the transmitter. It is known to use a bank of capacitors having a plurality of capacitors arranged in either series or parallel combinations to control the tuning of the coil of the LCR circuit. The capacitors may be switched into the circuit as needed.
Additionally, the use of a high inductance and high Q LCR circuit results in an increase in the voltage across the coil. Such increased voltage often necessitates the use of insulation of the coil to avoid or reduce partial discharge and corona discharges. Further, regulatory requirements may limit the amplitude of the output current, which may increase the cost of the circuit to meet the requirements. Thus, a low number of coil turns are needed to reduce the voltage. However, in high power applications, a high number of coil turns are needed to reduce the current. These opposing constraints result in a limitation of the amount of magnetic field that may be produced from, for example, an antenna having such an LCR circuit. It is known to provide a hybrid series-parallel resonant circuit to maintain a high magnetic field strength while reducing the increase in inductance and voltage. This circuit is limited in that the circuit does not allow compensation, for example, for tolerances in antenna coil construction and/or installation environment variations.
Thus, these known circuits for tuning often result in either increased power requirements and/or increased cost. Further, these known circuits may not perform satisfactorily because of the inability to provide compensation for variations of changes that affect the circuit.